The invention relates to elastic expanded polymer foams and also a process for producing them by sintering a mixture comprising foam particles P1 and P2 composed of different thermoplastic polymers or polymer blends.
Polystyrene foams are rigid foams. Their low elasticity is a disadvantage for many applications, for example in the packaging sector, since only unsatisfactory protection of the packaged goods against impact and shock stress is possible and the foam moldings used as packaging material break even at a low deformation, resulting in the protective effect of the foam under renewed stress being lost. There have therefore already been attempts in the past to increase the elasticity of polystyrene foams.
Expandable polymer mixtures of styrene polymers, polyolefins and if appropriate compatibilizers such as hydrogenated styrene-butadiene block copolymers are known, for example, from DE 24 13 375, DE 24 13 408 or DE 38 14 783. The foams which can be obtained therefrom are said to have better mechanical properties, in particular better elasticity and lower brittleness at low temperatures, compared to foams composed of styrene polymers and also insensitivity to solvents such as ethyl acetate and toluene. However, the blowing agent retention capability and the foamability of the expandable polymer mixtures to low densities are not sufficient for processing.
WO 2005/056652 describes expanded polymer foam moldings having a density in the range from 10 to 100 g/l which can be obtained by fusion of prefoamed foam particles derived from expandable, thermoplastic polymer pellets. The polymer pellets comprise mixtures of styrene polymers and other thermoplastic polymers and can be obtained by melt impregnation and subsequent pressurized underwater pelletization.
Elastic expanded polymer foams produced from expandable interpolymer particles are also known (e.g. US 2004/0152795 A1). The interpolymers can be obtained by polymerization of styrene in the presence of polyolefins in aqueous suspension and form an interpenetrating network of styrene polymers and olefin polymers. However, the blowing agent quickly diffuses out of the expandable polymer particles, so that they have to be stored at low temperatures and display satisfactory foamability for only a short time.
WO 2008/050909 describes elastic expanded polymer foams composed of expanded interpolymer particles having a core-shell structure, where the core comprises a polystyrene-polyolefin interpolymer and the shell comprises a polyolefin. These expanded polymer foams have improved elasticity and cracking resistance compared to EPS and are used first and foremost as transport packaging or as energy absorber in automobile applications.
WO 2005/092959 describes nanoporous polymer foams which can be obtained from blowing agent-comprising, multiphase polymer mixtures having domains in the range from 5 to 200 nm. The domains preferably comprise a core-shell particle which can be obtained by emulsion polymerization and in which the solubility of the blowing agent is at least twice as high as in the adjoining phases.
A new class of thermoplastic expanded polymer foams which have cells having an average cell size in the range from 20 to 500 μm and in which the cell membranes have a nanocellular or fibrous structure having pore or fiber diameters below 1500 nm has been described in WO 2008/125250.
The known cracking-resistant foams, for example derived from expanded polyolefins, expanded interpolymers or expandable interpolymers, are generally incompatible with or have only low compatibility with prefoamed, expandable polystyrene (EPS) particles. In processing to produce moldings such as foam blocks, poor fusion of the different foam particles is frequently observed.